I think with a 2.5g, ghost shrimp really would be your best bet. Betta's already get from 1.5-2.5 inches so you pretty much have your inch per gallon rule taken up... Although with bioload, the ipg rule sometimes doesn't even matter >.>... It's more of a guideline... Snails, as far as I'm concerned, it does depend on what kind you get- the bigger they are, the more you have to feed them and the more bioload it will cause. A nerite snail seems like it would be an okay bet, but still iffy because they get about an inch long, BUT it would also depend on how often you feed them and how much they eat... the more food you have in there the more extra waste you add to the tank. I think if you get a snail, it would be best to add alot more live plants in your tank that are low maintenance and eat ammonia and such like nobody's business... Of course, this is just my personal opinion. I've never dealt with having a tank as small as this with a snail, nor have I dealt with nerite snails, but I have my own betta rooming with an apple snail in a 5g that's planted. It's actually a bit less than 5g because I've added plant substrate, but the plants in there help alot- ammonia and nitrite are at 0ppm and nitrate is less than 5ppm (waiting on it to get to 0 though because the plants will eat all the nutrients that the nitrate eat).
Shrimp have a smaller bioload, so it wouldn't be as much of a hassle. However, you'd still want to increase your foliage if you don't plan on buying more shrimp for your betta to possibly snack on. With limited space like a 2.5g, your betta will definitely have more of a chance finding them... if anything, they're cheap and a good source of protein. Some members on here even breed shrimp for their bettas to snack on every once in a while.